A control device for optical rain sensors for use in vehicle windshield wiper systems is described in (U.S. Pat. No. 4,859,867), which includes a compensation circuit for ambient light, temperature and other system variables. To compensate for the ambient light, the optical transmitter (emitter device) of the rain sensor is operated with high-frequency pulsing. A receiver detects the high-frequency radiation output by the transmitter and supplies modulated signals to a microcontroller, which separates the alternating component (cyclical portion) of the signals from the direct component (non-cyclical portion) of the false signals (such as of ambient light, etc.). The frequency currently used for driving the transmitter, in this case a light-emitting diode (LED), is 1000 Hz. The preferred pulse width is 15 microseconds. The signals analyzed in the microcontroller control the wiper motor. As a secondary effect, the service life of the LED is prolonged as compared to continuous operation.
The service life is not sufficiently prolonged, however, when this high pulse frequency is used.
When the frequency is reduced and the turn-on time of the optical transmitter is kept the same to maximally prolong the service life, it becomes more and more difficult, however, to isolate the alternating component from the direct component. Moreover, the sensor's reaction time to the occurrence of moisture is adversely affected.